Self-interference cancellation for full-duplex communication using a phase and gain adjusted transmit signal

ABSTRACT

The present disclosure is directed to an apparatus and method for cancelling self-interference caused by full-duplex communication. In a full-duplex communication device, the receiver will generally experience significant self-interference from the full-duplex communication device&#39;s own transmitter transmitting a strong outbound signal over the same channel that the receiver is to receive a weak inbound signal. The apparatus and method are configured to adjust a phase and gain of the outbound signal provided at the output of a power amplifier (PA) and inject the phase and gain adjusted outbound signal at the input of a low-noise amplifier (LNA) to cancel the interference from the outbound signal in the inbound signal.

TECHNICAL FIELD

This application relates generally to self-interference cancellation,including self-interference cancellation for full-duplex communication.

BACKGROUND

A duplex communication system includes two transceivers that communicatewith each other over a channel in both directions. There are two typesof duplex communication systems: half-duplex communication systems andfull-duplex communication systems. In half-duplex communication systems,the two transceivers communicate with each other over the channel inboth directions but only in one direction at a time; that is, only oneof the two transceivers transmits at any given point in time, while theother receives. A full-duplex communication system, on the other hand,does not have such a limitation. Rather, in a full-duplex communicationsystem, the two transceivers can communicate with each other over thechannel simultaneously in both directions.

Wireless communication systems often emulate full-duplex communication.For example, in some wireless communication systems two transceiverscommunicate with each other simultaneously in both directions using twodifferent carrier frequencies or channels. This scheme, wherecommunication is carried out simultaneously in both directions using twodifferent carrier frequencies, is referred to as frequency divisionduplexing (FDD). FDD is said to only emulate full-duplex communicationbecause FDD uses two half-duplex channels rather than a single channelto accomplish simultaneous communication in both directions.

Although emulated full-duplex communication using FDD allows forsimultaneous communication in both directions, it requires two channels.True full-duplex communication eliminates the need for one of these twochannels, resulting in increased spectrum efficiency. The difficultywith true full-duplex communication, and the reason why it has notbecome common place in wireless and mobile communication standards todate, is the significant interference that the receiver of a full-duplexcommunication device will generally experience from the full-duplexcommunication device's own transmitter transmitting over the samechannel that the receiver is to receive signals. This interference isreferred to as self-interference because the interference experienced bythe receiver originates from its own paired transmitter.

For example, in some communication systems, signals can be transmittedat power levels as high as 25 dBm and signals can be received at powerlevels as low as −100 dBm. At these levels, the self-interference needsto be reduced by at least 25 dBm−(−100 dBm)=125 dBm to allow forinformation to be recovered from the received signals.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present disclosure and, togetherwith the description, further serve to explain the principles of thedisclosure and to enable a person skilled in the pertinent art to makeand use the disclosure.

FIG. 1 illustrates a block diagram of an example RF front-end configuredto emulate full-duplex communication using FDD.

FIG. 2 illustrates a block diagram of an example RF front-end configuredto emulate full-duplex communication using FDD.

FIG. 3 illustrates a block diagram of an example RF front-end configuredto perform true full-duplex communication in accordance with embodimentsof the present disclosure.

FIG. 4 illustrates another block diagram of an example RF front-endconfigured to perform true full-duplex communication in accordance withembodiments of the present disclosure.

FIG. 5 illustrates a block diagram of an example MIMO RF front-endconfigured to perform true full-duplex communication in accordance withembodiments of the present disclosure.

The present disclosure will be described with reference to theaccompanying drawings. The drawing in which an element first appears istypically indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the disclosure. However, itwill be apparent to those skilled in the art that the disclosure,including structures, systems, and methods, may be practiced withoutthese specific details. The description and representation herein arethe common means used by those experienced or skilled in the art to mosteffectively convey the substance of their work to others skilled in theart. In other instances, well-known methods, procedures, components, andcircuitry have not been described in detail to avoid unnecessarilyobscuring aspects of the disclosure.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of this discussion, the term “module” shall be understoodto include software, firmware, or hardware (such as one or morecircuits, microchips, processors, and/or devices), or any combinationthereof. In addition, it will be understood that each module can includeone, or more than one, component within an actual device, and eachcomponent that forms a part of the described module can function eithercooperatively or independently of any other component forming a part ofthe module. Conversely, multiple modules described herein can representa single component within an actual device. Further, components within amodule can be in a single device or distributed among multiple devicesin a wired or wireless manner.

1. OVERVIEW

The present disclosure is directed to an apparatus and method forcancelling self-interference caused by full-duplex communication. In afull-duplex communication device, the receiver will generally experiencesignificant self-interference from the full-duplex communicationdevice's own transmitter transmitting a strong outbound signal over thesame channel that the receiver is to receive a weak inbound signal. Theapparatus and method are configured to adjust a phase and gain of theoutbound signal provided at the output of a power amplifier (PA) andinject the phase and gain adjusted outbound signal at the input of alow-noise amplifier (LNA) to cancel interference from the outboundsignal in the inbound signal. In one embodiment, the apparatus andmethod use a passive network of resistors and capacitors to generate thephase and gain adjusted outbound signal. Before further describing theseand other features of the present disclosure, some of the currentself-interference suppression techniques are first described below.

2. CURRENT SELF-INTERFERENCE SUPPRESSION TECHNIQUES

The problem of self-interference is not unique to true full-duplexcommunication systems. For example, emulated full-duplex communicationsystems using FDD similarly suffer from self-interference, althougheffectively to a lesser degree due to the use of separate channels foreach direction of communication. Provided below is a description of someof the specific techniques used in emulated full-duplex communicationsystems to cope with self-interference, as well as a high-leveldescription of some of the self-interference suppression techniquesdeveloped specifically for true full-duplex communication systems.

Referring now to FIG. 1, a block diagram of an example RF front-end 100configured to emulate full-duplex communication using FDD isillustrated. RF front-end 100 includes an antenna 105, a duplexer 110, alow-noise amplifier (LNA) 115, and a power amplifier (PA) 120. Inoperation, RF front-end 100 transmits outbound signals and receivesinbound signals simultaneously over different channels (i.e. carrierfrequencies). For example, as illustrated in FIG. 1, both inbound andoutbound signals are simultaneously coupled between antenna 105 andduplexer 110 over a common signal path 130. The inbound signals arereceived at a carrier frequency f_(RX) that is different from thecarrier frequency f_(TX) at which the comparatively stronger outboundsignals are transmitted.

In such an arrangement, duplexer 110 is used to couple common signalpath 130 to both the input of LNA 115 and to the output of PA 120.Duplexer 110 provides the necessary coupling, while preventing thestrong outbound signals, produced by PA 120, from being coupled to theinput of LNA 115. In general, despite the fact that the outbound signalsand the inbound signals are transmitted over different carrierfrequencies, duplexer 110 is needed because the strong outbound signalscan still saturate LNA 115, leading to gain compression.

As illustrated in FIG. 1, duplexer 110 is a three-port device having anantenna port 135, a transmit port 140, and a receive port 145. Antennaport 135 is coupled to transmit port 140 through a transmit band-passfilter, included in duplexer 110, and to receive port 145 through areceive band-pass filter, further included in duplexer 110. The passband of the transmit filter is centered within the frequency range ofthe outbound signals, which are received at node 150 from a transmitter(not shown). The pass band of the receive filter is centered within thefrequency range of the inbound signals, which are passed to a receiver(not shown) at node 155. The transmit and receive band-pass filters areconfigured such that their respective stop bands overlap with eachother's pass bands. In this way, the band-pass filters isolate the inputof LNA 115 from the strong outbound signals produced by PA 120, as wellas the output of PA 120 from the received inbound signals. In typicalimplementations, duplexer 110 must attenuate the strong outbound signalsby about 50-60 dB to prevent the outbound signals from saturating LNA115.

Although effective for RF front-ends that transmit outbound signals andreceive inbound signals over different, non-overlapping portions of aparticular frequency band, this frequency selective method of isolationgenerally will not work for RF front-ends that implement truefull-duplex communication. This is because in true full-duplexcommunication the transmitted outbound signals and the received inboundsignals overlap in frequency and therefore cannot be isolated from oneanother in this regard.

FIG. 2 illustrates a block diagram of another RF front-end 200configured to provide emulated full-duplex communication. Unlike RFfront-end 100, illustrated in FIG. 1, which provides isolation usingfrequency selection, RF front-end 200 provides isolation usingelectrical balance. RF front-end 200 includes an antenna 205, anisolation module 215, a balancing network 220, an LNA 225, and a PA 230.In operation, RF front-end 200 transmits outbound signals and receivesinbound signals simultaneously over different channels or carrierfrequencies. For example, as illustrated in FIG. 2, both inbound andoutbound signals are simultaneously coupled between antenna 205 andisolation module 215 over a common signal path 235. The inbound signalsare received at a carrier frequency f_(RX) that is different from thecarrier frequency f_(TX) at which the comparatively stronger outboundsignals are transmitted.

In such an arrangement, isolation module 215 is used to couple commonsignal path 235 to a differential input 240 of LNA 225 and to an output245 of PA 230. Isolation module 215 provides the necessary coupling,while preventing strong outbound signals that are provided by PA 230from saturating LNA 225. Despite the fact that the outbound signals andthe inbound signals are transmitted over non-overlapping portions of aparticular frequency band, isolation module 215 is needed for the samereason RF front-end 100 in FIG. 1 needs duplexer 110: because the strongoutbound signals can saturate LNA 225, leading to gain compression.

Isolation module 215 is specifically implemented as a four-port devicehaving an antenna port 250, a transmit port 255, a differential receiveport 260, and a balance port 265. Isolation module 215, in conjunctionwith balancing network 220, is configured to isolate transmit port 255from differential receive port 260 by electrical balance. Specifically,the current of the strong outbound signals provided by PA 230 attransmit port 255 is split by isolation module 215, with a first portionof the current directed towards antenna 205 for transmission, and asecond portion of the current directed towards balancing network 220,where its associated energy is dissipated (as heat). In the idealsituation, balancing network 220 is configured to provide an impedancesubstantially equal to that of antenna 205 such that the first portionand second portion of current are equal (i.e., each are exactly one-halfthe total current of the strong outbound signals sourced by PA 230) andresult in equal voltages on the differential ends of LNA 225. In thisway, isolation module 215 can effectively isolate differential input 240of LNA 225 from the strong outbound signals.

Although electrical balance can be used in a similar manner to isolatethe strong output signals from the weak inbound signals in RF front-endsthat implement true full-duplex communication, up to half of the signalenergy produced by PA 230 is dissipated by balance network 220. As aresult, the outbound signals provided by PA 230 suffer a dissipationloss of around 3 dB (or half).

Current self-interference suppression techniques developed specificallyfor true full-duplex communication systems attempt to provide requiredhigh-levels of self-interference suppression using complex designs thatinclude, for example, one or more additional transmit antennas toeffectively perform spatial multiplexing or a series of fixed delaylines and variable attenuators to recreate the self-interference forcancelling the actual self-interference in the received signal. Theseapproaches are not only overly complex, but are often not practical formany applications, such as mobile wireless devices that have small formfactors.

3. SELF-INTERFERENCE CANCELLATION USING A PHASE AND GAIN ADJUSTEDOUTBOUND SIGNAL

FIG. 3 illustrates a block diagram of an example RF front-end 300configured to perform true full-duplex communication in accordance withembodiments of the present disclosure. RF front-end 300 can be used inany wireless transceiver, including those for cellular and wirelesslocal area network communications. RF front-end 300 includes an antenna305, an optional circulator 310, a self-interference cancellation module315 controlled by a digital signal processor (DSP) 320, a LNA 325, and aPA 330.

In operation, RF front-end 300 transmits outbound signals and receivesinbound signals simultaneously over the same channel or carrierfrequency. For example, as illustrated in FIG. 3, both inbound andoutbound signals are simultaneously coupled between antenna 305 andoptional circulator 310 over a common signal path 335. The inboundsignals are received at a carrier frequency f_(RX) that is the same asthe carrier frequency f_(TX) at which the comparatively strongeroutbound signals are transmitted (i.e., f_(TX)=f_(RX)).

In such an arrangement, optional circulator 310 is used to couple commonsignal path 335 to both the input of LNA 325 and to the output of PA330. Optional circulator 310 provides the necessary coupling, whilepreventing to some degree the strong outbound signals, produced by PA330, from being coupled to the input of LNA 325. In the case offull-duplex RF front-end 300, optional circulator 310 is used to notonly help prevent the strong outbound signal from saturating LNA 325,but also to help prevent the strong outbound signal from directlyinterfering with the weak inbound signal that it overlaps with infrequency.

As discussed above, the difficulty with true full-duplex communication,and the reason why it has not become common place in wireless and mobilecommunication standards to date, is the significant interference thatthe receiver of a full-duplex communication device will generallyexperience from the full-duplex communication device's own transmittertransmitting over the same channel that the receiver is to receivesignals. For example, in some communication systems, signals can betransmitted at power levels as high as 25 dBm and signals can bereceived at power levels as low as −100 dBm. At these levels, theself-interference needs to be reduced by at least 25 dBm−(−100 dBm)=125dBm to allow for information to be recovered from the received signals.

Although optional circulator 310 can help to reduce self-interference inthe inbound signal coupled to the input of LNA 325, optional circulator310 can generally only provide around 15 to 25 dB of isolation, which istoo low for many operating conditions to allow for full-duplexcommunication with a spectral efficiency gain over emulated full-duplexcommunication using FDD.

Self-interference cancellation module 315 can help to further bridgethis gap. More specifically, self-interference cancellation module 315is configured to adjust a phase and gain of the outbound signal,provided at the output of PA 330, and inject the phase and gain adjustedoutbound signal at the input of LNA 325 to cancel interference from theoutbound signal in the inbound signal. The phase of the outbound signalcan be specifically adjusted to effectively invert the outbound signaland delay the outbound signal to match the delay of the interferencefrom the outbound signal in the inbound signal at the input of LNA 325.The gain can be adjusted to match the magnitude of the interference fromthe outbound signal in the inbound signal at the input of LNA 325. DSP320 can specifically be used to set and dynamically adapt the value ofthe phase and gain adjustment provided by self-interference cancellationmodule 315.

It can be shown that, for channels as wide as 70 MHz, self-interferencecancellation module 315 can provide over 35 dB of additional isolation.It can be further shown that self-interference cancellation module 315can be fully implemented on a monolithic integrated circuit (IC) 350together with DSP 320, LNA 325, and optionally PA 330.

Referring now to FIG. 4, another block diagram of an example RFfront-end 400 is illustrated in accordance with embodiments of thepresent disclosure. RF front-end 400 has a similar configuration as RFfront-end 300 but includes a specific implementation ofself-interference cancellation module 405 and a matching network (MN)410. In general, the output of PA 330 is often differential and coupledto antenna 305 via matching network 410, which is used to help increasethe transfer of power from PA 330 to antenna 305 and reduce reflectionsfrom antenna 305 back towards PA 330.

As shown in FIG. 4, self-interference cancellation module 405 includesfour passive elements and four switches. In particular,self-interference cancellation module 405 includes two variableresistors R1 and R2 that are each controllably coupled between the inputof LNA 325 and a respective differential end of PA 330 via a switch. Inaddition, self-interference cancellation module 405 further includes twovariable capacitors C1 and C2 that are each controllably coupled betweenthe input of LNA 325 and a respective differential end of PA 330 via aswitch.

DSP 320 can independently adjust the resistances of resistors R1 and R2,the capacitances of capacitors C1 and C2, and the configuration of thefour switches (i.e., whether each switch is opened or closed) to adjustthe phase and gain of the outbound signal, provided at the output of PA330, and inject the resulting phase and gain adjusted outbound signal atthe input of LNA 325 to cancel interference from the outbound signal inthe inbound signal. The phase of the outbound signal can specifically beadjusted to effectively invert the outbound signal and delay theoutbound signal to match the delay of the interference from the outboundsignal in the inbound signal at the input of LNA 325. The gain can beadjusted to match the magnitude of the interference from the outboundsignal in the inbound signal at the input of LNA 325.

As will be appreciated by one of ordinary skill in the art,self-interference cancellation module 405 provides full flexibility interms of phase adjustment of the outbound signal. For example, assumingthat the top most differential end of PA 330 provides the outboundsignal at +90 degrees and the bottom most differential end of PA 330provides the outbound signal at −90 degrees, one of the following fourswitch configurations can be used based on the desired phase shift:

-   -   1. For a desired phase shift between 0-90 degrees, R1 and C1 can        be coupled between LNA 325 and PA 330 by closing their        respective switches and R2 and C2 can be decoupled between LNA        325 and PA 330 by opening their respective switches.    -   2. For a desired phase shift between 90-180 degrees, R2 and C1        can be coupled between LNA 325 and PA 330 by closing their        respective switches and R1 and C2 can be decoupled between LNA        325 and PA 330 by opening their respective switches.    -   3. For a desired phase shift between 180-270 degrees, R2 and C2        can be coupled between LNA 325 and PA 330 by closing their        respective switches and R1 and C1 can be decoupled between LNA        325 and PA 330 by opening their respective switches.    -   4. For a desired phase shift between 270-360 degrees, R1 and C2        can be coupled between LNA 325 and PA 330 by closing their        respective switches and R2 and C1 can be decoupled between LNA        325 and PA 330 by opening their respective switches.

Referring now to FIG. 5, a block diagram of a multiple-input,multiple-output (MIMO) RF front-end 500 is illustrated in accordancewith embodiments of the present disclosure. In MIMO RF front-end 500,two single-antenna RF front-ends 505 and 510 are used to perform amulti-antenna technique, such as beamforming or spatial multiplexing.MIMO RF front-end 500 can be used in any wireless transceiver, includingthose for cellular and wireless local area network communications.

As shown in FIG. 5, single antenna RF front-ends 505 and 510 each have asubstantially similar structure as RF front-end 300 in FIG. 3, but eachincludes an additional self-interference cancellation module for everyother RF front-end within MIMO RF front-end 500. Because exemplary MIMORF front-end 500 includes only two, single antenna RF front-ends 505 and510, each single antenna RF front-end 505 and 510 includes only oneadditional self-interference cancellation module.

More specifically, single antenna RF front-end 505 includes aself-interference cancellation module 515 that is used in the samemanner as self-interference cancellation module 315 in FIG. 3 to adjusta phase and gain of the outbound signal, provided at the output of itsown PA 525, and inject the phase and gain adjusted outbound signal atthe input of its own LNA 530 to cancel interference.

In addition, RF front-end 505 includes a self-interference cancellationmodule 535 that is used to adjust a phase and gain of the outboundsignal, provided at the output of its own PA 525, and inject the phaseand gain adjusted outbound signal at the input of LNA 545 to cancelinterference. Self-interference cancellation module 535 can specificallyadjust the phase of the outbound signal, provided at the output of PA525, to effectively invert and delay the outbound signal to match thedelay of the interference from the outbound signal in the inbound signalat the input of LNA 545. Self-interference cancellation module 535 canfurther adjust the gain of the outbound signal, provided at the outputof PA 525, to match the magnitude of the interference from the outboundsignal in the inbound signal at the input of LNA 545.

Single antenna RF front-end 510 includes a self-interferencecancellation module 520 that is used in the same manner asself-interference cancellation module 315 in FIG. 3 to adjust a phaseand gain of the outbound signal, provided at the output of its own PA540, and inject the phase and gain adjusted outbound signal at the inputof its own LNA 545 to cancel interference.

In addition, RF front-end 510 includes a self-interference cancellationmodule 550 that is used to adjust a phase and gain of the outboundsignal, provided at the output of its own PA 540, and inject the phaseand gain adjusted outbound signal at the input of LNA 530 to cancelinterference. Self-interference cancellation module 550 can specificallyadjust the phase of the outbound signal, provided at the output of PA540, to effectively invert and delay the outbound signal to match thedelay of the interference from the outbound signal in the inbound signalat the input of LNA 530. Self-interference cancellation module 550 canfurther adjust the gain of the outbound signal, provided at the outputof PA 540, to match the magnitude of the interference from the outboundsignal in the inbound signal at the input of LNA 530.

It should be noted that MIMO RF front-end 500 is provided by way ofexample and not limitation. Other MIMO RF front-ends 500 can includemore than two single antenna RF front-ends as would be appreciated by aperson of ordinary skill in the art. It should be further noted that oneor more DSPs can be used to control self-interference cancellationmodules 515, 520, 535, and 550 similar to DSP 320 described above inregard to FIG. 3. Finally, it should be noted that each ofself-interference cancellation modules 515, 520, 535, and 550 can beimplemented similar to self-interference cancellation module 405 in FIG.4 using a network of passive elements and switches.

4. CONCLUSION

Embodiments have been described above with the aid of functionalbuilding blocks illustrating the implementation of specified functionsand relationships thereof. The boundaries of these functional buildingblocks have been arbitrarily defined herein for the convenience of thedescription. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the disclosure that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent disclosure. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

What is claimed is:
 1. An apparatus comprising: a power amplifier (PA)configured to provide, at an output of the PA, an outbound signal at acarrier frequency to an antenna; a low-noise amplifier (LNA) configuredto receive, at an input of the LNA, an inbound signal at the carrierfrequency from the antenna; and a self-interference cancellation moduleconfigured to adjust a phase and gain of the outbound signal provided atthe output of the PA and inject the phase and gain adjusted outboundsignal at the input of the LNA to cancel interference from the outboundsignal in the inbound signal, wherein the self-interference cancellationmodule is configured to adjust the phase of the outbound signal providedat the output of the PA by controllably coupling, using switches,passive elements between the input of the LNA and differential ends ofthe output of the PA in different combinations.
 2. The apparatus ofclaim 1, wherein the self: interference cancellation module isconfigured to adjust the phase and gain of the outbound signal providedat the output of the PA based on a phase and gain of the interferencefrom the outbound signal in the inbound signal.
 3. The apparatus ofclaim 1, wherein the self-interference cancellation module includes aresistor and a capacitor.
 4. The apparatus of claim 3, wherein theresistance of the resistor and the capacitance of the capacitor areconfigured to be adjusted to adjust the gain and phase of the outboundsignal.
 5. The apparatus of claim 1, wherein the self-interferencecancellation module includes: a first resistor controllably coupledbetween a first differential end of the output of the PA and the inputof the LNA, a first capacitor controllably coupled between the firstdifferential end of the output of the PA and the input of the LNA, asecond resistor controllably coupled between a second differential endof the output of the PA and the input of the LNA, and a second capacitorcontrollably coupled between the second differential end of the outputof the PA and the input of the LNA.
 6. The apparatus of claim 5, whereinthe resistances of the first and second resistors and the capacitancesof the first and second capacitors are configured to be adjusted toadjust the gain and phase of the outbound signal.
 7. The apparatus ofclaim 5, based on the phase of the interference from the outbound signalin the inbound signal, coupling at most one of the first resistorbetween the first differential end of the output of the PA and the inputof the LNA or the second resistor between the second differential end ofthe output of the PA and the input of the LNA.
 8. The apparatus of claim5, based on the phase of the interference from the outbound signal inthe inbound signal, coupling at most one of the first capacitor betweenthe first differential end of the output of the PA and the input of theLNA or the second capacitor between the second differential end of theoutput of the PA and the input of the LNA.
 9. The apparatus of claim 1,further comprising a circulator coupled between the antenna and theoutput of the PA and coupled between the antenna and the input of theLNA.
 10. An apparatus comprising: a first self-interference cancellationmodule configured to inject a phase and gain adjusted version of a firstoutbound signal, provided at an output of a first power amplifier (PA),at an input of a low-noise amplifier (LNA) to cancel interference fromthe first outbound signal in an inbound signal; and a secondself-interference cancellation module configured to inject a phase andgain adjusted version of a second outbound signal, provided at an outputof a second PA, at the input of the LNA to cancel interference from thesecond outbound signal in the inbound signal, wherein the first outboundsignal, the second outbound signal, and the inbound signal are all at asame carrier frequency.
 11. The apparatus of claim 10, wherein the firstself-interference cancellation module is configured to adjust the phaseand gain of the first outbound signal based on a phase and gain of theinterference from the first outbound signal in the inbound signal. 12.The apparatus of claim 10, wherein the first self-interferencecancellation module includes a resistor and a capacitor.
 13. Theapparatus of claim 12, wherein the resistance of the resistor and thecapacitance of the capacitor is configured to be adjusted to adjust thegain and phase of the first outbound signal.
 14. The apparatus of claim10, wherein the first self-interference cancellation module includes: afirst resistor controllably coupled between a first differential end ofthe output of the first PA and the input of the LNA, a first capacitorcontrollably coupled between the first differential end of the output ofthe first PA and the input of the LNA, a second resistor controllablycoupled between a second differential end of the output of the first PAand the input of the LNA, and a second capacitor controllably coupledbetween the second differential end of the output of the first PA andthe input of the LNA.
 15. The apparatus of claim 14, wherein theresistances of the first and second resistors and the capacitances ofthe first and second capacitors are configured to be adjusted to adjustthe gain and phase of the first outbound signal.
 16. An apparatuscomprising: a power amplifier (PA) configured to provide, at adifferential output of the PA, an outbound signal at a carrier frequencyto an antenna; a low-noise amplifier (LNA) configured to receive, at aninput of the LNA, an inbound signal at the carrier frequency from theantenna; and a self-interference cancellation module configured tocancel interference from the outbound signal in the inbound signal atthe input of the LNA, the self-interference cancellation modulecomprising: a first resistor controllably coupled, using a first switch,between a first differential end of the output of the PA and the inputof the LNA, a first capacitor controllably coupled, using a secondswitch, between the first differential end of the output of the PA andthe input of the LNA, a second resistor controllably coupled, using athird switch, between a second differential end of the output of the PAand the input of the LNA, and a second capacitor controllably coupled,using a fourth switch, between the second differential end of the outputof the PA and the input of the LNA.
 17. The apparatus of claim 16,wherein the self-interference cancellation module is configured toadjust the phase and gain of the outbound signal based on a phase andgain of the interference from the outbound signal in the inbound signal.18. The apparatus of claim 16, wherein the resistances of the first andsecond resistors and the capacitances of the first and second capacitorsare configured to be adjusted to adjust the gain and phase of theoutbound signal provided at the output of the PA.
 19. The apparatus ofclaim 16, based on the phase of the interference from the outboundsignal in the inbound signal, coupling at most one of the first resistorbetween the first differential end of the output of the PA and the inputof the LNA or the second resistor between the second differential end ofthe output of the PA and the input of the LNA.
 20. The apparatus ofclaim 16, based on the phase of the interference from the outboundsignal in the inbound signal, coupling at most one of the firstcapacitor between the first differential end of the output of the PA andthe input of the LNA or the second capacitor between the seconddifferential end of the output of the PA and the input of the LNA.